It's not even stereotyping or racism, it's simple and justified prejudice.
Almost all adult monsters we've seen in the show are warriors/mercs/bandits who make a living by threatening Mewman lives or livelihoods. Why the monsters do it is irrelevant.
Your average monster IS far more dangerous than the average Mewman. Even if they're a perfectly civilized person, it's completely natural for a Mewman to be wary of a creature that could easily kill them and looks the part.
The popular talking point these days is that "oppressors" like the Mewmans are "in a position of power," but the Mewman's position is secured entirely by their Queen's magic. Toffee understood this, which is why he targeted that power before his second attack. It's been demonstrated time and time again that when monsters organize they can easily threaten the kingdom and more than once have only been driven back when the castle itself is stormed. The Knights of Mewni can barely put up a fight, nevermind a family of peasants on the outskirts of town.
Then you'd have people who would say "well the Mewmans took this land by force hundreds of years ago, so the monsters of today are right to take it back" which is a completely garbage take that the show has never even once entertained.